The invention relates to an improved device for measuring the rate of flow of fluid through a conduit, and more particularly, to a rotating vane meter which measures mass flow rate directly without requiring density compensating subsystems.
It is frequently necessary in an industrial setting to measure the flow of fluids through pipes. For example, in bulk processing of compounds in chemical plants the amount of various fluids being introduced into a reaction vessel must be determined and controlled. In plant operations requiring pressurized air or steam it is often desirable or necessary to monitor consumption of these fluids. In other instances, the supplier of a working fluid or fuel desires to know the quantity of fluid delivered in order that an appropriate fee may be charged.
A number of fluid flow measuring devices are currently known. The most commonly used fluid flow metering devices can be generally categorized as belonging to one of three groups. Positive displacement meters function by receiving and discharging discrete volumes of fluid through, for example, a reciprocating piston in a cylinder. The number of cycles of such a device occurring in a unit period of time is proportional to the flow rate of fluid passing through the meter. Although accurate, positive displacement meters are mechanically complex and are highly sensitive to foreign matter contamination. Obstruction type meters employ an orifice or other restriction in the fluid path and the flow rate is calculated from the measured pressure drop across the restriction. Those meters generally have limited measurement ranges and are highly sensitive to the flow patterns of the fluid passing therethrough. Moreover, obstruction meters provide instantaneous flow measurement which result must be integrated to evaluate total flow. Rotating vane type meters are frequently used in measuring fluid flow. These meters function by causing the flowing fluid to impart a tangential force on an impeller causing rotation thereof. The rotational velocity of the impeller is related to volumetric flow rate. A rapidly spinning impeller, however, produces inaccuracies since it cannot react rapidly to flow rate changes due to rotational inertial effects.
The major disadvantage inherent in each of the types of fluid flow meters described above is the fact that they measure volumetric flow rate rather than mass flow rate. The mass of fluid flowing rather than the volumetric flow rate is often a more significant measurement. Meters which measure volumetric flow rate can frequently be compensated through incorporation of a fluid density sensing mechanism in order that the device measure mass flow rate. This is possible since mass flow rate is the product of volumetric flow rate and fluid density. Fluid density is determined by sensing fluid temperature and pressure. The provision of additional subsystems to correct the meter for mass flow rate requires the incorporation of a number of elements to the meter and thereby tends to increase cost, complexity and negatively affects reliability. More typically, however, currently available flow meters simply ignore density variations in instances where extreme variations in density are not anticipated and relate volumetric flow rate to mass flow rate directly by assuming a certain density. This approach, however, produces errors even when minor density variations are encountered.
Rotating vane type meters such as described above are very widely used since they are relatively inexpensive, reliable and provide total flow measurement. An effort to reduce the impeller inertial effects of rotating vane type meters, reduce bearing wear, simplify impeller balancing and reduce complexity of and wear on the readout device led designers of meters according to the prior art to slow down the rotational speed of the impeller by providing a resistive torque. This feature further was believed to produce a meter which would relate mass flow rate directly with impeller rotational speed. It has been found, however, that meters designed according to these principles do not accurately reflect fluid density variations and hence were not true mass flow meter. The present invention describes an improved rotating vane type meter which overcomes the above-described shortcomings in that accurate mass flow readings are provided.
The meter according to this invention employs a driving axial flow multi-bladed impeller to which a torque is imparted due to fluid reaction against the impeller blades. Resisting rotation of the driving impeller is a balancing impeller which rotates in a fluid such that a linear torque versus rotational speed relationship results. Unlike rotating vane meters according to the prior art, this improved meter measures mass flow rate directly. This result occurs primarily by drastically slowing down the rotational speed of the driving impeller such that a maximum speed at rated capacity is approximately 40 rpm. This compares to the impeller rotational speed of 500 to 1,000 rpm for meters designed according to the prior art. In the development of the present invention, it was found that the effect of the impeller blades moving away from the fluid flowing through the blades was a primary cause of the inaccuracies of previous rotating vane meters in measuring mass flow rate. By drastically slowing the impeller blades, it was discovered that performance approximating that of a completely stalled impeller can be achieved. Meters employing completely stalled vanes are known, but have disadvantages which are avoided by my invention, particularly with respect to the accuracy and complexity of the vane force measuring and readout system.
A second embodiment of a mass flow meter is disclosed herein in which two driving impellers are employed in such manner that the axial loads imposed on each are opposing and ingestion of liquids into the meter will not result in high structural loads on the meter components.